1. Field of the Invention
The present invention relates to electronic devices such as a data compression apparatus and a data decompression apparatus, which are used for compressing and decompressing data including various data such as image data and audio data, and an electronic device and system including at least one of these data compression apparatus and data decompression apparatus. Furthermore, the present invention pertains to a data compression method and a data decompression method using the electronic devices and the system, and a data compression program and a data decompression program for controlling the electronic devices and the system.
2. Background-Art
The Joint Photographic Expert Group (JPEG) standard is the international standard for image compression technology that compresses still images. JPEG algorithms can be largely divided into two compression methodologies. The first methodology is based on the discrete cosine transform (DCT), and the second methodology is a spatial domain method performing the differential PCM (DPCM) in the two-dimensional image space.
The DCT is the basis for an algorithm adapted for the present invention and can acquire sufficient decoded image quality even with a small number of bits although it has a lossy coding characteristic of generally not perfectly reconstructing an original image. On the other hand, the spatial domain method is a function added as a standard for implementing a lossless coding characteristic of perfectly reconstructing an original image with low compressibility. The DCT is further divided into a baseline process (baseline system), which is an essential function, and an extended DCT process (extended system), which is an optional function. The baseline process is an algorithm based on the adaptive discrete cosine transform coding (ADCT), and is the required minimum function for all coders/decoders implementing the DCT. Image compression in the baseline process is carried out for each image data block of 8×8 pixels.
Currently, as such, most of the standard digital image compression formats are based on DCT in block units. Compressibility by definition is dependent on convergence rate of DCT coefficients since these methods are based on the cosine series approximation. In general, compressibility decreases when the convergence of DCT coefficients is slow. Furthermore, quantization error of the DCT coefficients increases as coding rate (bit rate) decreases, and deterioration of reconstructed images such as blocking artifact cannot be ignored.